The present invention relates generally to the art of making bags from a plastic film. More specifically, it relates to a method and apparatus for folding and separating bags.
There are many uses and designs for plastic bags. Such bags are typically manufactured from plastic films, and there are many known machines for automatically making bags from such a film. Some bag making machines create bags on a continuous strip of plastic film or web (typically a flattened tube or a continuous folded sheet. Bags are made by forming seals (typically transverse to the machine direction or along the side of the film). Adjacent bags are separated from one another by forming a perforation parallel to (and preferably close to) the seal. The perforation allows the bags to be separated (either manually or in a downstream device).
Bag making machines often include equipment that separates adjacent bags and then folds and stacks the separated bags. Bags are separated in some prior art machines by operating a downstream nip at a higher speed than the speed at which the upstream film or web is travelling. When the perforated film or web encounters the downstream rollers (often called separation rollers), the higher speed of the rollers pulls the web, thereby tearing along the perforations. The separation rollers are typically mounted on a cam, cylinder, or some other device, which intermittently brings the separation rollers into contact with each other and the film, to separate adjacent bags. This sort of separation was adequate at lower speeds, but it often limited the operating speed.
An example of the intermittent contact type of prior art separator is found in U.S. Pat. No. 5,388,746 issued Feb. 14, 1995. This is a complex design and the oversped rolls are operated at a constant speed, thus the available control is limited.
Many prior art separators include an infeed nip formed by driven rolls. This results in a linear speed difference between the ropes and the rolls. Prior art separators also often include a zone where the bag is not between ropes, as the separated bag passes from the separator to the downstream section (such as a folder). The bags are not controlled at that gap, and can jam the machine. Additionally, prior art machines typically have ropes which are returned by rolls in the film path. Thus, a pinch point that can catch the film or bag is created between the rope and the roll. This can create jams in the machine.
After the bags have been separated it is common for them to be provided to a folder that folds the bags one or more times. Generally, a folder includes a number of folding sections, wherein the maximum number of folds available is equal to the number of folding stations.
U.S. Pat. No. 5,388,746, issued Feb. 14, 1995, shows a prior art folder. The folder shown therein includes three folding sections located downstream of a separator. The separated bags are traveling in a downward direction as they approach each folding station. The leading edge of the bag passes a folding nip, and then the middle of the bag is blown in a direction almost perpendicular to the original path direction (close to horizontal). The bag is grabbed in a nip and then folded. The bag continues on in a substantially horizontal direction until it approaches the second folding station. Then, the bag must be redirected in a downward direction where the second folding station operates substantially as did the first folding station. A third folding station (also mounted such that the bag must enter it in a downward direction) is also provided.
The arrangement shown in U.S. Pat. No. 5,388,746 is relatively complex, and the bags make two turns for each fold. The bags are travelling in one direction entering the folding point, and exit the folding station in a different direction. Thus, the momentum of the trailing end of the bag is not useful in helping to crease the new fold bag because of the different directions. The extra turns in the path of the bags also make handling the bags more difficult and more likely for jams or other failures of the process.
Moreover, the path of the return ropes is such that access to the folding stations for service, adjustment etc. is relatively difficult. Also, because the bags are not held by ropes or nips immediately prior to the folding point, there is an opportunity for a bag to be skewed or improperly folded.
After the bags have been folded they typically are slowed down so that they may be more easily managed in downstream processing stations. One prior art method for slowing down folded bags is a passive system, wherein the bags enter a slow down section, and are allowed to gradually slow down to rope speed. One problem with this passive slowing down is that the bags can easily become skewed from the machine direction, and it is hard to properly control the bag speed.
Another prior art slow down section includes a downward discharge. The bag falls in the downward discharge into two belts that form a V. The apex of the V is a nip such that the bag falls into that nip and is slowed down by the speed of the belts. One problem with this arrangement is that the bag can bunch up as it enters the nip.
Another prior art method is shown in U.S. Pat. No. 4,073,223, issued Feb. 14, 1978. This method uses a rudimentary form of control wherein a pair of rollers are operating at a slower speed than the upstream machine speed. The rollers have bars mounted thereon, arranged such that when the rollers rotate, the bars come in contact with one another. The rollers are turned such that momentary contact is made between the bars and the trailing edge of the bag, thereby slowing the bag down. One problem with this system is that the rotation of the rollers is a continuous motion system which is not indexed to each bag. Thus, it is difficult to maintain the proper timing over a long period of operation of the machine. Also, there are other timing related problems which occur at certain speeds and bag lengths.
Accordingly, it is desirable for a separator and folder to include a separator that properly separates adjacent bags. Additionally, the separator should be designed such that bags are not likely to flutter as they leave the separator. Preferably, such a folder and separator should also provide for continuous holding of separated bags to prevent fly back.
The folding section should preferably be configured without unnecessary turns so as to avoid unnecessary complexity and cost. Preferably, it should be designed such that in the event less than the maximum number of folds is being implemented the bag does not have to undergo turns. Improperly folded bags should be discharged in downward direction so they do not cause jams. The folding section should be easily accessible from the top. Also, the folding section should utilize the momentum of the bag prior to the folding to help fold the bag.
A slow down section preferably includes a slow down nip that is easy to control and can be precisely aligned with the bags.
According to a first aspect of the invention a folder, and method for folding, plastic bags includes at least one folding station. The folding station has a folding point at which an input portion, an output portion, and a storage portion meet. The bag travels in an input direction in the input portion, in an output direction in the output portion, and in a storage direction in the storage portion. The input direction is different from the storage direction.
In one embodiment the output direction is substantially the same as the input direction. In another the input direction is substantially horizontal. The storage direction has a downward component in an alternative embodiment. The storage direction is preferably less than 30xc2x0 from vertical.
A turning air source is disposed to direct the bag into the storage portion in another embodiment. A folding air source is disposed to direct the bag into the output section in another embodiment. A fin is disposed to help guide the bag into the output portion in an alternative. A turning roll is disposed above the turning point, and imparts motion to the bag, generally in the storage direction, in yet another embodiment.
The folder includes a second folding station downstream of the first folding station in another embodiment. The second folding station is configured like the first folding station.
The output direction for the first station is the same as the input direction for the second station, and the bag path between the first and second folding stations is substantially linear in another embodiment.
The folder includes a separator located upstream of the folder in another alternative. The separator includes a separator nip formed by separator rolls that are in, and remain in, the film path in one embodiment. The separator rolls are preferably servo-driven rolls.
The separator includes guides, such as ropes or belts, that guide the bag through at least a part of the separator. The folder includes guides, such as ropes or belts, that guide the bags through at least one part of the folder. The separator guides overlap the folder guides. The guides have return rolls located out of the film path in another embodiment.
The folder includes a slow-down section located downstream of the folding station in an alternative. The slow-down-section preferably includes two servo-driven rolls, each with at least one projection thereon, disposed to contact the bags. In one embodiment each servo-driven roll has two projections.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description and the appended claims.